The present invention relates to a method for the treatment of wastewater and, in particular, to such a method for use with a system of sequencing batch reactors.
In the early history of wastewater treatment by microorganisms, the wastewater was often batched and treated by various processes of agitation, aeration or the like. With the amount of wastewater to be treated increasing in volume and in impurities, batch treatment became fairly labor intensive and was eventually substantially replaced by continuous wastewater treatment processes in the 1920's and 1930's.
However, with the relatively recent innovation of computers which can be programmed to control valves, motors, etc. in the wastewater treatment process system, batch reactors again appear to be a viable alternative and offer attractive advantages over continuous processes.
A sequencing batch reactor system generally incorporates a series of batch reactors, usually two or three which use a sequence of steps to treat wastewater. Each of these reactors retains a certain amount of activated sludge and allows for the removal of excess sludge. The activated sludge contains microorganisms which assist in the breakdown of waste materials when provided with adequate oxygen levels.
Each reactor in a sequencing batch reactor system operates in a cyclical process. During the cycle for a particular reactor, the reactor must complete the process of treating a batch of wastewater. The batch wastewater treatment process includes a fill period, a react period, a settle period and a decant period. An idle period may also be included in the treatment process.
During the fill period, wastewater is introduced into a batch reactor. The fill period can be further divided into an anoxic fill period and an aerated fill period. During the anoxic fill period, wastewater is introduced into the batch reactor without aeration and during aerated fill, the wastewater already introduced into the batch is aerated while the reactor continues to fill, thereby providing oxygen to the microorganisms in the activated sludge.
At the end of the fill period, incoming wastewater is diverted to another batch reactor which then begins its cycle. The just filled reactor then enters the react period wherein the wastewater contained in the reactor is aerated for a predetermined time period. Aeration of the contents of the batch reactor results in the mixing of the activated sludge and the wastewater as well as the introduction of oxygen into this mixture. The introduction of oxygen into the mixed wastewater and sludge is required by the microorganisms contained in the sludge to effect the decomposition of various wastewater components, including biodegradable organic matter.
In the aeration of batch reactors, a set of pumps is often utilized which recirculates the mixed wastewater and sludge throughout the system and which forces oxygen into the resulting mixture. The use of this pumping system places the greatest energy demands on the system of any step in the batch reactor sequence.
At the end of the predetermined aeration period, the system enters a settle period where quiescent conditions are maintained. These quiescent conditions allow the reactor contents to separate into a clarified effluent layer and a sludge layer. After separation is complete, the sludge layer rests on the bottom of the reactor and the clarified effluent layer is located above the sludge layer. The effluent layer is subdivided into a lower buffer volume and an upper decant volume.
At the end of the settle period, the decant period begins and the decant volume of the clarified effluent is removed from the reactor. The decant volume is normally equal to the volume of influent received during the previous fill period. However, the decant volume and, therefore, the fill volume is limited to a maximum volume based on the dimensions of the reactor. The buffer volume is retained in the reactor during the decant period and provides a buffer zone between the sludge layer and the decant volume to reduce the possibility of sludge uptake during the decanting process.
At the end of the decant period, the reactor typically enters an idle period until each reactor of the system has sequenced through the filling cycle after which wastewater is directed back to that reactor and the reactor begins its cycle again with the fill period.
The length of certain of these steps has been varied for various reasons and, in particular, to respond to varying influent flow rates.
It would be possible to operate a sequencing batch reactor system so that the reactors only treated full batches by having the fill period last until the reactor filled by an amount equal to the normal decant volume. However, this mode of operation would result in extremely long cycles for reactors at low influent flow rates. During these long cycles, reactors which were not being filled would sit idle for extended periods of time. The extended idle periods would be harmful to microorganism populations in the sludge and would result in inefficient use of the reactors. In order to optimize reactor use and maintain microorganism population, conventional operating strategies generally establish a maximum cycle time for the reactors.
Based on the maximum cycle time, the conventional operating strategy is time based with level overrides. Timers are set for the fill period, the anoxic fill period, the aeration period (aerated fill period plus aerated react period) and the settle period. The fill timer is set for a period equal to the maximum cycle time divided by the number of reactors. For example, in a two reactor system having a six hour maximum cycle time, the fill timer for each reactor would be set at three hours. The influent flow rate required to fill the reactor or, more specifically, the maximum decant volume within the time set on the fill timer is the design flow rate. The maximum cycle time is generally picked so that the reactor operates a majority of the time at flow rates near or below the design flow rate.
At flow rates below the design flow rate, the reactor will not fill completely before the fill timer expires, therefore, the reactor is allowed to fill until the fill the next reactor which will begin its fill period. At flow rates above the design flow rate, the reactor will fill completely before the timer expires, triggering a level override. The level override causes the influent flow to be diverted to the next reactor ending the fill period in one reactor and beginning it in the next. In conventional batch reactor systems, at flow rates below the design flow rate, the amount of time allowed for the anoxic fill period, the aerobic fill period, the react period and the settle period remain constant, despite changes in the influent flow rate and the batch size. By maintaining constant aeration times for varying sized batches, conventional operation strategies either unnecessarily waste large amounts of energy by aerating small batches too long, or provide inadequate aeration for larger batches.
At flow rates above the design flow rate, the time available for the non-filling reactors to go through the various wastewater treatment steps becomes a limiting factor in the wastewater treatment strategy. As the influent flow rate increases, the fill time decreases, thereby decreasing the time available for the non-filling reactor(s) to complete all the waste treatment steps.
Because the time required for the settle period and the decant period is generally constant for full batches and the fill period is determined by the influent flow rate, conventional operating strategies generally compensate for the narrowing time constraints by reducing the idle period and then the react period. The react period can be initially reduced without reducing the overall aeration time by providing for an aerated fill period, although this does reduce anoxic time. Then, as the time constraints narrow, the aerated fill period is continually increased and the react period is decreased. This will reduce the anoxic fill period while maintaining a constant aeration time. Eventually, the react period is eliminated and all the aeration takes place during the fill period. The anoxic fill period is consequently eliminated. Elimination of the anoxic fill period and the react period (as opposed to an aerobic fill period) is undesirable. Each of these steps is important for the effective decomposition of waste material by microorganisms contained in activated sludge.